struct ListNode* FindKthToTail(struct ListNode* pListHead, int k) {
  struct ListNode* slow = pListHead;
  struct ListNode* fast = pListHead;
    while(k--){
        if(!fast)
            return NULL;
        fast = fast->next;
    }
    while(fast){
        fast = fast->next;
        slow = slow->next;
    }
    return slow;
}